Aptamer coated measurement and reference electrodes and methods using same for biomarker detection

ABSTRACT

A device for identifying the presence of a specific target molecule or biomarker by the detection of a change in an electrical property includes a measurement sensor  8  comprising a semiconducting sensor structure  12  capable of conjugating with the biomarker, thus giving rise to the said change in electrical property, and an electrode system  3, 4  for conducting a signal from the device. According to the invention there is a further such sensor  9 , of substantially identical form but having its sensor structure  14  already conjugated with the biomarker, or otherwise capped, e.g. using a further oligonucleotide strand, so as to act as an internal reference. When a biological sample, e.g. saliva, is applied to the electrodes, the reference enables the discounting of all environmental effects other than the biomarker. The invention provides a simple, cheap and accurate text for one or more biomarkers that can be used in the field without complex equipment.

This invention relates to the detection of chemical, organic andbiological analytes. More specifically, the invention relates tobiomarker detection and is concerned with a device for reliablydetecting the presence of low concentrations of molecular biomarkers insamples of fluid.

A promising field of application for such detection is in identifyingdiseases by way of Biomarker molecules. Existing approaches to thediagnosis of chronic diseases (for example tuberculosis) often rely onintricate laboratory tests on samples of blood, urine or tissue takenfrom a patient. In the exemplary case of tuberculosis (TB), the use of a‘sputum smear’ test is common, relying on the identification ofMycobacterium tuberculosis bacilli using a microscope, or alternativelya tuberculin skin test.

It will be appreciated that it is not usually possible to carry out suchintricate tests in close proximity to the patient, and therefore anetwork of laboratories is required, equipped with expensive testingequipment and highly trained staff. A complex logistics network isrequired to transport samples from the patient testing centres to theplace of testing, and to deliver the results back to the patients aftera delay.

It is known that chronic diseases such as tuberculosis are prevalent indeveloping countries, where the conditions described previously forsuccessful testing are difficult to provide. Testing equipment might betoo expensive for there to be an effective network of laboratories forthe size of population or there could be a lack of skilled personnel towork in the laboratories. The transport of samples between thelaboratories and the point of care could be difficult, and even upondelivery of a test result back to the point of care, it could provedifficult to find infected patients after intervals of up to severalweeks, particularly in a transient population.

Insufficient access to the most advanced tests mean that tuberculosisdetection programmes in disease-endemic countries are reliant onantiquated and inaccurate methods such as smear microscopy, solidculture, chest radiography, and skin testing. In the case of the legacytuberculosis tests mentioned above, it is known that the tuberculin skintest has the disadvantage of being unable to distinguish between thelatent and active stages of tuberculosis. The sputum smear test is onlyaccurate in one half to three quarter of cases, requiring a large numberof organisms in the sample and a skilled microscope operator able todistinguish between M. tuberculosis and other mycobacteria.

Molecular biomarkers offer an attractive new method of diseasedetection, compared with known techniques involving centralisedlaboratory testing. The rapidly advancing field of proteomicsdemonstrates that many diseases can be differentiated by testing for thepresence of molecular biomarkers in bodily fluids such as blood plasma,urine or saliva.

One biomarker relevant to the detection of tuberculosis is neopterin, acatabolic product synthesised by macrophages (white blood cells) uponstimulation by the signalling molecule gamma-interferon. The presence ofneopterin is known to indicate an inflammatory immune response, and onedisease that causes production of neopterin is tuberculosis. Othermacrophage activation markers are known, such as procalcitonin,C-reactive protein, soluble intercellular adhesion molecule 1, solubleurokinase plasminogen activator receptor, and Monocyte CDIIc66.

Another biomarker relevant to the detection of tuberculosis is thrombin,a coagulation factor which acts on fibrinogen to produce fibrin, afibrous protein involved in the clotting of blood which also appears atheightened levels in patients suffering from tuberculosis. It will beappreciated that there exists a large and growing range of molecularbiomarkers useful for disease diagnosis, and that the two biomarkersmentioned previously are examples. Other examples are Lysozyme andNicotinamide adenine dinucleotide (NAD).

It will also be appreciated that the expression of different biomarkerscan vary in dependence on the stage of the disease. For example,neopterin levels are increased at diagnosis based on the extent of thedisease, and they decrease during and after treatment. A subsequentincrease in neopterin levels is associated with relapse. Consistentquantitative longitudinal measurements are therefore of great diagnosticuse.

Other biomarkers can be identified in microbial markers in sputum,microbial markers in urine, tuberculosis-specific T-cell function, andother macrophage activation markers. Indications of reactivation risk,and the eradication of a latent infection of tuberculosis, can also bepredicted using for example gamma-interferon, or neopterin.

Biomarkers can be used to judge vaccine efficacy, for example bymonitoring polyfunctional T-cells.

Increased test specificity and predictive value can be achieved bycombining an ensemble of non-specific biomarkers related to tuberculosisby measuring multiple parameters resulting from proteomics (thelarge-scale study of proteins), metabolomics (the study of chemicalfingerprints that specific cellular processes leave behind), andtranscriptomics (the study of mRNA used in gene transcription).

One way of identifying biomarkers is by the use of DNA-like molecules,as is shown for instance in WO 2007/001401 (Dupont/Boussard et al.),where oligonucleotides on carbon nanotubes are used. Aptamers aresynthetic oligonucleotides (a short nucleic-acid polymer), ligands orpeptides that can be isolated or created using for example the SELEXprocess against targets as diverse as small organic modules, toxins,bacterial and viral proteins, virus-infected cells, cancer cells andpathogenic organisms. They have defined shapes, and bind functionalsites on their respective targets with affinities and specificities thatoften exceed those of the much more widely developed antibody reagents.Nucleic acid aptamers are easily isolated by a semi-automated totally invitro process, removing the need for animal experiments. Furthercharacterisation allows them to be minimised so that they can be made onthe gram scale by synthetic chemical routes. They are easilyconcatenated with other nucleic acid sequences allowing bifunctionalspecies to be created. Chemical functionalization of the aptamers toallow straightforward immobilisation and detection is also trivial. Itis possible to synthesize an aptamer functionalized to conjugate to oneof the disease-indicating biomarkers discussed above, such as thrombinor neopterin.

In order to expose aptamers to a solution possibly containing targetmolecules, they need to be bound to or “mounted on” a suitablesubstrate. To detect a binding event, it is advantageous if thesubstrate is conductive or semiconductive, and if it has a largespecific surface area. One promising substrate is carbon nanotubes.

Carbon nanotubes are well known allotropes of carbon with a cylindricalstructure. The aptamer may be attached to the nanotubes using eithercovalent or non-covalent approaches. For example, it is possible toattach a chemical “foot” structure to the non-biomarker-specific end ofthe aptamer molecule using a compound such as anthracene, which willthen make contact with the surface of a carbon nanotube, coating it witha layer of biomarker-specific aptamer. In particular when the carbonnanotubes have semiconducting properties, when a carbon nanotube iscoated with an aptamer and then exposed to the analyte that the coatedaptamer binds to, the large number of binding events and the change inconductivity of the aptamer-coated carbon nano-wire or nanotube will bedetectable by electronic means, for example by detecting a change inconductivity, capacitance, impedance, or inductance, potentially underhigh-frequency alternating current. Such aptamer binding events can alsoaffect the conductivity of metallic nanotubes.

Such aptamer-coated carbon nanotubes as described in the previousparagraph can be applied across the gate of a Field Effect Transistor,in effect forming the channel. A carbon-nanotube FET (CNT-FET) withsource, drain and back-gate contact, with the channel domain made ofaptamer-coated carbon nanotube strands, is discussed in U.S. Pat. No.7,854,826 (So et al./Korea Research Institute of Chemical Technology).

Although such detectors are promising, they are prone to variation intheir accuracy and sensitivity. This is due to individual variation,variation in electrolyte concentration, temperature swings, geometry andother factors.

It will be appreciated that the chemical characteristics of the bodilyfluids containing biomarkers of interest will vary; between differentindividuals, differing concentrations of electrolytes and othermolecules such as proteins and enzymes in bodily fluids will be found.Without a method of controlling for these spurious effects,distinguishing for the presence of biomarkers is likely to beunreliable.

It is desirable to increase the reliability of sensors of this type, andin particular to provide a biomarker detector which can be used in thedifficult context previously described to provide rapid and reliabledisease detection in an inexpensive manner, and in a way that does notrequire the presence of skilled laboratory personnel.

SUMMARY OF THE INVENTION

According to the invention there is provided a device as claimed inclaim 1 and a method as claimed in claim 26.

The invention uses a referencing system to reduce the likelihood offalse biomarker detection events. Embodiments of the invention use anaptamer-coated conducting or semiconducting base or support, such as anarray or deposit of carbon nanotubes, or a sheet-like structure such asgraphene, or semiconducting DNA or other “nanowires” to form a sensorstructure, and a second sensor structure that is essentially identicalbut is “capped”, that is, the aptamer is prevented from recognising thetarget molecules, e.g. by being pre-coated with or conjugated or boundto the target molecules, or by being conjugated with a complementary DNAstrands, or by being a mutant version differing in a few bases. Examplesof conjugation may, amongst others, include the use of standardphoto-cross-linking of the aptamer sequence to its target, or retentionof a protective complementary oligo on the aptamer strand; one may alsouse an aptamer sequence variant that prevents binding. In the presenceof target molecules or diagnostic biomarkers that bind to the aptamer,electrons are transferred and a change in conductivity of the supportoccurs. The semiconducting base could be said to act as the channel of aField-Effect Transistor or CNT-FET. This change can be detected bysuitable electronic circuitry, which may be built into the device,connected to the electrode system. The reading is then taken bycomparing the “live” sensor with the pre-conjugated (reference) sensor.Since the two sensors of the pair are in identical molecularenvironments, i.e. inhabit the same measurement space, other variationse.g. resulting from electrolytes in the fluid samples are cancelled out.

Aptamers are a preferred option as a reasonably specific and cheap formof target-molecule detector, but other biomarker-receptive molecules orstructures are envisaged, even antibodies or peptide aptamers forinstance, provided that a sufficient electrical signal results from thebinding process.

Often one wishes to measure several biomarkers simultaneously, becausesuch an ensemble of biomarkers can more accurately indicate the presenceof a chronic condition such as tuberculosis; in some embodiments,therefore, the detector has several such pairs of sensors and can easilybe used to detect multiple biomarkers simultaneously and reliably.

More specifically, each of the biomarker-receptive detectors will alsobe placed next to, for instance, a biomarker-saturated detector. Suchreference detectors will be identical to the biomarker detectors, withthe important difference the reference electrodes will use aptamerswhich will already have been bound onto their target, e.g. byphoto-cross-linking, use of a non-binding sequence variant or byretention of a base paired complementary strand, so that they are‘capped’ to ensure that they do not react during a diagnostic test. Thatis, the aptamer is pre-bound with the target molecule or biologicalspecies. Apart from this difference, the reference detector is analysedfor a change in conductivity, capacitance, etc. in the same manner asfor the active biomarker detector. The purpose of this is that themultiple biomarker detectors, for example five in number, each now havean internal reference of a positive analyte detection event, subject tothe same spurious effects that affect the entire detector. In this way,a ‘control’ comparison is provided in order to allow spuriousfirst-order effects due to the variability between individuals orsampling conditions to be removed and thereby to calibrate the detectionof each biomarker.

Various methods for coating supports with aptamers exist. As analternative to the case with a carbon nanotube (or graphene) coated withbiomarker-specific aptamer, it is possible first to functionalise a DNAstrand—used purely as a support—with aptamer, and then to wind thefunctionalised DNA strand around a carbon nanotube substrate (or thestrand can be wound first and then functionalised). Thebiomarker-specific aptamer and the capped aptamer can be attached to theDNA groups using click chemistry. Another possibility is to attach theaptamer to the nanotubes via a streptavidin-biotin linkage, where one ofthese elements is functionalised onto the aptamer and the other elementis attached to the nanotubes via a covalent or non-covalent bond.Alternatively, a metallised DNA nano-wire could be coated directly withaptamer and capped aptamer. Another alternative could use conductingpolymer coated with the aptamer or capped aptamer.

As mentioned previously, the binding of target biomarkers to theirspecific aptamers will cause a small but detectable change in theelectrical characteristics across the electrodes. If need be, thissignal can be amplified by making the electrodes the source-drainterminals of a CNT-FET, for instance. After a sample has been applied tothe multiple biomarker-specific detectors and their accompanyingreferences, the CNT-FETs are interrogated to determine the change intheir electrical characteristics. Such measurements could take the formof a simple conductivity test with a DC source, or more complicateddetermination of impedance at certain frequencies or over a range offrequencies. The same test would be applied to the multiple referencedetectors, allowing the spurious background effects present at the“live” detector to be removed from the genuine biomarker concentrationmeasurement.

In one kind of construction, gold source and drain contacts of a CNT-FETare arranged as an etched interdigitated pattern on a silicon substrate,with the aptamer-coated carbon nanotubes laid over the top to form akind of channel or semiconducting bridge. When using carbon nanotubeswhich have a mixture of conducting natures, it is preferable that thespacing of the interdigitated pattern is greater than the length of anindividual nanotube to prevent the electrodes being shorted by onemetallic nanotube acting as an impurity.

An alternative detection arrangement uses a polymer substrate, with goldelectrode pairs laid out in a gridiron pattern and the aptamer-coatedcarbon nanotubes laid over the interdigitated pattern. Many variationsare possible, as is known; in each case, pairs of sensor structures areused, with appropriate circuitry, to give a referenced result.

To perform the measurement process and obtain the measurements discussedabove automatically, circuitry to interface with the CNT-FET array,including for example switches, signal sources, amplifiers,analogue-to-digital converters and microprocessors, is provided.

To ensure that there is sufficient test biological fluid, e.g. sputum,urine, blood etc., present on the sensor head before a measurement,logic circuitry is provided to provide a built-in self-test capability.Upon initial activation, the change of conductivity of each detector'sreference electrode is recorded and compared against a known range, toensure that sufficient fluid has been applied to the detector, and thatsubsequent measurements will be valid. The circuit performs a dryconductivity measurement on all tracks to verify the system's integrity.After the application of for example saliva or other biological fluid tothe detector, the change in conductivity is verified to ensure that itis above a predefined level. Amplifying circuitry will be used to ensurethat the dynamic ranges of the electrical signals are appropriate, foreach sensor element. For some fluids, pretreatment in a suitable bufferwould be followed by selective filtration, e.g. to remove cellcontaminants.

In order to satisfy the stated need for an inexpensive, rapid,point-of-care diagnostic sensor which can be operated by minimallytrained personnel, the entire arrangement of detection substrate andcircuitry discussed previously can be mounted inside a moulded bodyresembling known digital camera memory cards, for example. The sensorarray is contained within a shallow trough to allow samples of bodilyfluid to make contact with the aptamer-coated CNT-FET array. Adetachable sealing film covers the active area of the detector toprevent the ingress of contaminants. It is envisaged that one side orend of the body moulding will connect the detection circuitry to amobile handheld device such as a smart-phone, using one of the many dataconnectors and protocols available. In another embodiment the sensormoulding is inserted into a solution as a “dip stick” to measure otherbodily fluids such as urine. Because of the small size of the detector,the mechanical support afforded by the socket should hold the detectioncard in a stable manner.

It will be appreciated that results generated by the detector can bedownloaded onto a smart-phone or other handheld device, for collection,display or analysis. Additionally, the data can be forwarded via anyaccessible commodity radio transmission equipment and TCP/IP stackcontained within the smart-phone to a centralised server.

BRIEF SUMMARY OF THE DRAWINGS

For a better understanding of the invention, and to show how it may becarried into effect, reference will now be made, by way of example, tothe accompanying drawings in which:

FIG. 1 is a side view of an embodiment having a moulded body containingthe substrate, sensors and sealable film;

FIG. 2 is a plan view of the layout of an arrangement of reference andsensor electrodes;

FIG. 3 is a plan view of an embodiment with five reference-sensorelectrode pairs lying on a substrate;

FIG. 4 provides end-on and plan schematic views of the way in which anexemplary aptamer is attached to a carbon-nanotube backbone;

FIG. 5 shows three different embodiments of aptamer-biomarkerarrangement on a DNA backbone;

FIG. 6 shows the different arrangement of capped and uncapped biomarkeras applied to the DNA backbone of the reference and sensor electrodes;

FIG. 7 shows a sequence of steps using click chemistry to form thefunctionalised CNTs;

FIG. 8 shows a control system connected to the sensors, for furtherprocessing the results and transmitting information to and from theprocessing means;

FIG. 9 shows atomic force microscopy of single-walled carbon nanotubeswhich have been dispersed using single-strand herring sperm and thendried on a substrate.

FIG. 10a shows a schematic of the inter-digitated electrodes used tomeasure the electrical properties of the nanotube networks, includingthose suitably functionalised to detect the target analyte.

FIG. 10b shows a schematic of an optical micrograph of the sensorinterdigitated electrodes.

FIG. 11 shows the electrode current as function of gate voltage fornanotube networks formed by the deposition of herring-sperm DNA wrappednanotubes.

FIG. 12 shows the electrical characteristics of a nanotube device madeby the deposition of nanotube-(GT)₁₀ complex.

FIG. 13 shows the device current (right axis, dark line) and theabsolute device current (left axis, light line) as a function of gatevoltage for “protected” and “unprotected” devices where the aptamer isagainst lysozyme;

FIG. 14 shows the device characteristics an embodiment with deprotected(GT)₁₀ thrombin aptamer;

FIG. 15 shows a characteristic for a setup as in FIG. 14 but with theprotecting group left in place;

FIG. 16a shows results from the as-made reference electrode (nanotubesfunctionalised by protected (GT)₁₀ thrombin aptamer), while FIG. 16bshows the deprotected result;

FIG. 17a shows results for the as-made reference electrode (nanotubesfunctionalised by protected (GT)₁₀ thrombin aptamer) and FIG. 17b showsthe deprotected equivalent.

DETAILED DESCRIPTION

A biomarker detection device 50 embodying the invention is mountedinside a body moulding 1 as illustrated schematically in FIG. 1. Asubstrate 2 is attached to the inside of the body moulding 1, and aplurality of planar contacts or electrodes 3, 4 is applied to the topside of the substrate to form sensors 8, 9. A shallow trough 6 can beformed to surround the contact areas of the substrate, to direct fluidsto the contact areas at the start of a test, and to enable fluids to beretained at the contact areas during the detection process, incircumstances where the amount of fluid is small. A removable polymersealing strip 7 is attached to the top surface of the body moulding witha weak adhesive, to provide a barrier seal against air and moisture inorder to prevent the contact areas of the substrate from becomingprematurely exposed to contaminants or being degraded by oxidation orhydrolysis.

The arrangement of the electrodes on the substrate 2 is shown in moredetail in the plan view of FIG. 2. The substrate 2 may consist ofsilicon, with preferably 300 nm thickness of silicon dioxide coating.For each sensor 8, 9, the electrodes 3, 4 are formed by alternatelydepositing and etching to leave parallel interdigitated contacts formedfrom a 20 nm thick chromium adhesion layer underneath a 100 nm thickgold layer. Typically, the linear tracks are spaced 10-50 μm apart andare connected alternately to electrode contacts 30, 31. These contactscould be labelled “source” and “drain”, though, strictly, in mostembodiments, the device is not really a FET. The interdigitatedelectrodes are alternated in perhaps twenty or thirty pairs (the drawingis schematic and shows only a few pairs), giving a total width ofperhaps 300 μm, and a comparable height (in the drawing). Terminals 10,11 collect current from the interdigitated electrodes 3, 4 respectively.The electrode pairs of each sensor may be placed in close proximity toeach other, say 500-1000 μm.

A conducting or semiconducting sensor structure 12 capable ofconjugating with a target biomarker of interest is coated onto the firstof the pair of interdigitated contacts 3, 4, forming what will bereferred to as the “measurement” sensor. The further sensor structure 14of substantially identical nature, but already conjugated with thebiomarker targeted by the measurement sensor is applied to the second ofthe pair of interdigitated contacts 9, forming what will be referred toas the reference sensor. When interfaced with appropriate electroniccircuitry to be described subsequently, the pair of measurement andreference sensors together forms a detection pair 16 for one biomarkerof interest.

The sensor structure 12 capable of conjugating with the target biomarkercomprises carbon nanotubes functionalized with a specific aptamercreated by coating the substrate 2 and therefore the interdigitatedcontacts 3, 4 with a layer of carbon nanotubes. The aptamer can be ashort (say about 40 nucleotides) length of DNA or RNA, or a peptidefragment, for instance. In the case where the substrate 2 is silicon, athin insulating layer is applied to the electrodes and a back-gatedcarbon-nanotube “FET” is formed having terminals which one may labelsource and drain, a channel formed by the carbon nanotubes, and a gateformed by a doped layer in the silicon. If amplification by back-gatingis not necessary, the gate need not be present.

In embodiments where a network of semiconducting nanotubes is used asthe sensor structure, the spacing of the electrodes should be designedto ensure that, even if a sample of semiconducting nanotubes containsconducting nanotube impurities, statistically there is unlikely to be apath of conducting nanotubes bridging the electrode, shorting thesensor. The CNTs may be of the order of 1-10 μm long if the tracks are10-50 μm apart. Their structure and function is described later in moredetail.

The spaces defined by the interdigitated electrodes in effect form avery wide channel between two electrodes. The collected signals from theelectrodes of each interdigitated pair are fed to suitable circuitry(not shown). Here the signal from the capped sensor 9 is compared withthe exposed or “measurement” sensor 8. Whereas the absolute level of thesignal can be expected to drift or vary with conditions, the difference(or ratio, or other comparison) gives a reliable reading. Since the twosensors are close together, they are affected equally by theirenvironment.

For certain applications, it is often necessary to detect at the sametime an ensemble of biomarkers, often referred to as a biosignature or“fingerprint”, for instance of a disease, to improve the likelihood of acorrect detection event. FIG. 3 illustrates a plan view of a biomarkerdetector similar to that in FIG. 2 but having a linear arrangement offive detection pairs each with its measurement 8 and reference 9 sensoron a common substrate 2, with their interdigitated electrodes shown veryschematically; it will be appreciated that a larger or smaller number ofdetection pairs could be deployed according to the precise ensemble ofbiomarkers of interest, which will vary with the intended application ofthe device. The linear array of detection pairs here coincides with anextended shallow trough 6 a, to ensure that fluid samples can becorrectly contained over the array of detection pairs.

In general, biomarkers are indicative of, but not 100% specific to, say,a disease. However, by obtaining quantitative readings of a suitable setof biomarkers a good degree of confidence can be achieved. For instance,TB may be identified using four biomarkers: Neopterin, which indicatesInflammatory processes and oxidative stress in cells; Procalcitonin,which distinguishes bacterial diseases as compared with viral diseases;Lipoarabinomannan (LAM), which tends to distinguish latent vs. activeTB, and C-Reactive Protein (CRP), which again tends to be associatedwith inflammatory processes and oxidative stress. See Tuberculosis 4,Biomarkers and diagnostics for tuberculosis: progress, needs, andtranslation into practice; Robert S Wallis, Madhukar Pai, Dick Menzies,T Mark Doherty, Gerhard Walzl, Mark D Perkins, Alimuddin Zumlat.Published Online May 19, 2010; DOI:10.1016/S0140-6736 (10) 60359-5:http://www.mossmanassociates.com/TB %20Biomarker %20report%20Lancet_2010.pdf

A device such as that shown in FIG. 3 can present a result in a singlereading.

As an alternative to this multiple individual measurement-and-referencesystem, multiple reference systems can be templated in a regular patternfor a single measure and reference result.

Alternative track layouts can be used for the detection area when it isfabricated on a plastic substrate, with the contact area formed from anumber of parallel track pairs arranged in a grid pattern. In such anembodiment, the contacts may be formed from deposited gold, silver orcarbon applied by an inkjet printing process. The interdigitated trackscan be straight or winding or convoluted.

To manufacture the back-gated or top-gated FET forming the semiconductorsensor structure 12, the channel domain between the source and draincontacts (spaced approximately 10 μm apart) must be coated with afunctionalized channel material, preferably CNTs, in particular SW(single-walled) CNTs, and preferably predominantly semiconducting,though these are more expensive. One simple way of achieving this end isto coat the electrodes with a solvent suspension of SWCNTs.

In one exemplary technique the SWCNTs are modified with β-Cyclodextrin(β-CD) which does not require prolonged heating, filtration, and washingof the CNTs, which can cause damage. The SWCNTs are dispersed in a β-CDsolution with sonication, typically resulting in a 2 mg/ml, suspension.An aliquot of the suspension is then applied to the contact area in thetrough 6 of the substrate 2. To ensure that the alignment of thefunctionalised SWCNTs 12, 14 across the interdigitated contacts 3, 4 isas uniform as possible, it is possible to apply an electric field acrossthe contact area of the substrate 2 as the aliquot evaporates, but themain requirement is that the nanotubes are uniformly distributed.

Another exemplary technique for accurately depositing carbon nanotubesbetween electrodes based on dielectrophoresis is given in Suhiro et al.(“Fabrication of a carbon-nanotube-based gas sensor usingdielectrophoresis and its application for ammonia detection by impedancespectroscopy”, J. Phys D: Appl. Phys. 36 (2003) L109-L114). In thismethod, multi-walled CNTs suspended in ethanol are sonicated. Aninterdigitated chromium electrode array fabricated upon a glasssubstrate is surrounded by a silicone-rubber spacer to form a sealedchamber. The carbon nanotube suspension is continuously circulated overthe electrodes, which are simultaneously excited by a 100 kHz, 10 Vpk-pk AC voltage, resulting in the multi-walled carbon nanotubesbridging the gaps between the electrode fingers. It will be appreciatedthat other methods of depositing carbon nanotubes between electrodescould be employed.

Following the application of the functionalised carbon nanotubes 12, 14to the contact area, they are subjected to a ‘pegylation’ treatment.Pegylation is known to those skilled in the art as the process by whichpolyethylene glycol (PEG) chains are attached to proteins, therebyincreasing the molecular mass of the proteins. However, in thisapplication, the polyethylene glycol is deposited directly onto thesurface of the carbon nanotubes, covering the areas of the nanotube thathave not been functionalised with an aptamer or a DNA strand. Theintention of this treatment is to ensure that proteins not specific tothe functionalised aptamer cannot bind to any unfunctionalised part ofthe sensor, thus introducing additional spurious effects into themeasurement. This has the effect of improving the sensitivity of afunctionalised nanotube when exposed to a fluid sample containing adiversity of protein species. The substrate 1 and/or the trough 6 mayalso be treated to pegylate their surfaces to prevent absorption ofspecies which may interfere with the measurement process. A typical PEGtreatment leaves a layer between 2 and 3 nm thick on the surface of thesensor. The process can be performed either on the substrate surfaceafter nanotube deposition, or in solution before the nanotubes arecoated onto the contacts.

It will be appreciated that the details of the fabrication of themeasurement 19 and reference 20 electrodes of a detection pair 16 areidentical, with the exception of the SWCNT coating applied to thecontacts. Therefore the fabrication of the measurement and referenceelectrodes could take place simultaneously until the later stage ofSWCNT coating. At this point, two separate solutions of SWCNTs areapplied, the first solution to the electrodes of the measurement sensor19, with the reference sensor masked, and the second solution to thereference sensor 20, with the measurement sensor masked. The solutionsdiffer in that the carbon nanotubes suspended in the first solutionapplied to the measurement electrode 19 are functionalised aptamerscapable of binding to a target biomarker. The carbon nanotubes containedin the second solution applied to the reference electrode arefunctionalised with an aptamer that is already saturated (pre-bound,conjugated or capped) with the target biomarker.

In operation, a device is prepared with a capped sensor structure 14 onone area of substrate with electrodes 4 and an uncapped or bare sensorstructure 12 on the other substrate area, with electrodes 3. The deviceis exposed to a solution to be measured, by immersion or application ofa drop of the analyte, such as a fluid. Electrolyte and other contentmay cause a change of conductivity of the sensor structures 12, 14 butthis is the same for both. Only binding events cause a differentialchange. The resulting signal is analysed by suitable circuitry of thedevice. A test can be run for a set time, such as 30 seconds, taking areading automatically at the end, so as to give a measure ofconcentration in the electrolyte.

One could also detect the target analyte (causative molecule) in air ornon-biological systems if the analyte is VX or similar.

A typical detection event will consist of measuring the doping such ashole or electron transfer that occurs in the binding event between theimmobilized aptamer on the surface of the carbon nanotube, and thetarget biomarker. A large number of such doping events (e.g. electrontransfers) caused by binding events will modulate the source-draincurrent flow in the “CNT-FET”, an effect that can be further influencedby the applied gate-voltage.

The device can also act as a time-averaging sensor in the manner of aradiation monitor; that is, it remains in place and keeps conjugating toavailable targets, and only displays a signal if the time averageconjugation, which may be related to concentration, is above a certainvalue, or it integrates over time to measure an exposure greater than athreshold.

As mentioned in relation to FIG. 3 and previously in the description, adevice with an array of sensor pairs will detect an ensemble ofbiomarkers, with each biomarker corresponding to a detection pair. Forexample, a detector able to detect five biomarkers will consist of asubstrate 2 with a row 26 of five detection pairs. For a devicetargeting five biomarkers, ten separate SWCNT coatings would be applied,the first set of five coatings representing the ‘live’ coated SWCNTsapplied to the measurement electrodes, and the second set of coatingsrelated to the capped-aptamer coated SWCNTs applied to the referenceelectrode.

As mentioned above, in order to detect specific molecules thecarbon-nanotube bases first need to be “functionalized”. One way ofdoing this is for single-walled carbon nanotubes (SWCNT) to be directlyfunctionalized by non-covalent attachment of biomarker-specific aptamersto the nanotubes' outer wall.

Preferably, aptamers are immobilised on the surface of the carbonnanotube using a substance with a high affinity for the carbon nanotube,for example anthracene or a single stranded DNA support strand (whichacts as a “foot”). The carbon nanotube is then said to be functionalizedto the biomarker targeted by the aptamer. The highly schematic FIG. 4provides axial and plan views of a carbon nanotube 27 with a genericfunctionalized aptamer 28 immobilised on the external wall of a carbonnanotube using anthracene 29. However, it will be appreciated that manymethods are known in the art for nanotube functionalisation, for examplecarboxylation followed by esterification, or click chemistry asdescribed below. It should be noted that the drawing is schematic, andin reality a CNT would be much longer than is implied, while aptamers inparticular are unlikely to be spherical.

A typical carbon nanotube might be 1000 nm long with a diameter of 1.25nm, and that the respective effective diameters of the aptamer and theanthracene are 3 nm and 0.1 nm. Under these assumptions, fourfunctionalized aptamers can be attached to a circumference of a SWCNT,and 330 along the length, leading to an estimate of a total of 1320functionalized aptamers per SWCNT.

In a second type of method of functionalizing carbon nanotubes, asingle-stranded DNA is modified by ‘click’ chemistry to supportaptamers, as shown in FIG. 5. This strand is then wound around asingle-walled carbon nanotube.

‘Click chemistry’ describes reactions between functional groups whichresult in a stable linkage, exhibit minimal thermal cross-reactivitywith other functional groups, react to completion, are free ofappreciable amounts of side products, and proceed under benign reactionconditions.

The click chemistry paradigm can be applied to modification of nucleicacids, and it is known that it can be used to label oligonucleotideswith fluorescent dyes, sugars or peptides; to cyclise DNA; and to joinoligonucleotides to DNA.

In a preferred embodiment the attachment consists of an polynucleotidefoot, preferably alternating GT sequences of (GT)_(n), where the numberof GT repeats, n, is 5 to 50, most preferably n=10. The GT sequenceincludes a pendant alkyne functional group onto which an azide-modifiedaptamer may be attached through ‘click’ chemistry. Most preferably, theaptamer is attached to the middle of the foot structure forming aT-junction to ensure good contact with the nanotube.

Alternatively, the carbon nanotube may be functionalised directly withthe aptamer using ‘click’ chemistry, in the absence of the intermediaryDNA wound around the carbon nanotube.

Preferably, in both the cases where the aptamer is attached to thepolynucleotide foot or directly onto the CNT, the aptamer is protectedduring the chemical conjugation by a complementary molecule, such as aDNA strand. Once the conjugation is complete, the protecting molecule isremoved. Such an approach reduces the undesired functionalisation of therecognition part of the aptamer during chemistry and reduces theprobability of undesired adsorption of the aptamer onto the nanotubes.The protecting sequence is sufficiently complementary to the aptamer toensure sufficient binding during the functionalisation so that theaptamer is protected but may contain mismatched sections in order toreduce the melting point of the complex to allow facile removal. Urea orpH changes may also be used during the removal process to reduce thetemperature required.

In order to saturate the surfaces of the nanotubes to prevent unspecificbinding of the target aptamers excess (GT)_(n) may be introduced.

FIG. 5 shows a DNA strand wound around a SWCNT or MWCNT, or otherconducting medium. Three alternative arrangements of aptamers attachedto a DNA backbone with ‘click’ chemistry are illustrated. In (a), abiomarker-specific aptamer is conjugated to each of the Adenine,Cytosine, Guanine or Thymine bases of a DNA strand. In (b) an aptamer isconjugated only to the Adenine base, for example, leading to a lowerdensity of aptamers compared to the backbone in (a). In (c) up to fouraptamers targeting different biomarkers have been conjugated with theDNA. It will be appreciated that the aptamers conjugated with thebackbone may be of the active variety (not conjugated with their targetbiomarkers), or may already be conjugated with their target biomarkers(capped). In this way, strands of DNA suitable for winding around carbonnanotubes forming either the measurement or reference sensors can beprepared.

A hybrid of the functionalized DNA and carbon nanotubes is formed bywinding the functionalized DNA strands around the nanotubes. It will beappreciated that either in the embodiment concerning direct attachmentof aptamers to a carbon nanotube, for example via carboxylation, or inthe embodiment describing the winding of a DNA strand around a carbonnanotube, the aptamers attached either directly to the nanotube or tothe DNA strand are in the measurement sensor unconjugated and in thereference sensor conjugated with their target biomarker or otherwisecapped (they are ‘uncapped’ or ‘capped’ respectively).

FIG. 6 is a diagram corresponding to FIG. 5(b) and showing a pair ofsensor structures, measurement and reference, with the referencestructure pre-bound with biomarker and the measurement structure havingsome biomarker bound to it during the measurement process.

The diagrams show aptamers bound to the bases, but in fact it is likelyto be the hydrophobic bases that bind to the CNT with the backbonefacing outwards, so that the aptamers would be bound to the backbone.

FIG. 7 shows a way of preparing the aptamer-DNA contains for the CNTs. ADNA strand (vertical) composed mostly of an alternating GT sequence,including a single alkyne-T adjacent to a short unique sequence, isprovided on the one hand, and on the other the desired aptamer DNA(horizontal) with 3′-azide modification, protected by a fullycomplementary DNA strand with the added base pairing region to align onthe supporting DNA strand (foot) and a 5′-biotin modification. Clickconjugation (second panel of FIG. 7) takes place at the alkyne-azidelocation on addition of Cu⁺. The DNA is then dispersed with CNTs andsuitably treated, e.g. by agitation, causing the CNTs to disperseevenly. Then the suspension is incubated with streptavidin coated ontomagnetic beads and heated briefly. The streptavidin binds to the biotin(panel 3) and the beads can then be removed (panel 4), pulling theprotective complement from the aptamers and making the active CNTs readyfor use.

Meanwhile the other part of the batch, for the reference sensors, keepsthe aptamer protection. The solutions are then dispersed over theinterdigitated electrodes. It is preferable to carry out the latter stepafter the aptamer binding, so that all possible sites are accessible forbinding. As shown in the final diagram, the effect of the target binding(left) can then be compared to the reference or inactive sensor (right)in identical chemical conditions.

In another embodiment, the sensor may be formed using the techniquescommonly associated with the area of printed electronics. In this case,a substrate may be formed from a plastic polymer, for examplepolyethylene terephthalate (PET), polyethylene naphthalate (PEN),poly-4-vinylphenol (PVP), or PEG. Tracks, of metal or carbon, aredeposited on the plastic substrate, and the carbon nanotubes aredeposited between them using the method described subsequently.

Referring again to FIG. 2, it will be understood that means formeasuring electrical characteristics between the interdigitated source30 and drain 31 contacts of the reference and measurement electrodesshould be provided. As described above, the measurement sensor 8 hasmain electrode terminals 30 and 31 and a gate, somewhat analogously to aFET, but the coating of aptamer-coated carbon nanotubes is appliedbetween the “source” and “drain” terminals to prepare the gate area ofthe FET. The terminals of the device can be connected to electronicstimulation 37 and measurement 38 means to measure the electricalcharacteristics between the source and drain electrodes.

For example, the stimulation means 37 could assume the form of a voltagesource, and the measurement means 38 could assume the form of anammeter, allowing an assessment of the conductivity variation betweenthe source and drain electrodes 30, 31.

It will be appreciated that other electronic circuitry such asinstrumentation amplifiers and variable current or voltage sources couldbe used during the measurement process. It will also be appreciated thatthe stimulation 37 and measurement 38 means could be fabricated onto thesame substrate 2 that forms the substrate for the contact area, if it issilicon, or alternatively could reside on a separate board in closeproximity to the silicon substrate, with the source 30 and drain 31contacts being connected by bond wire.

By changing the stimulation and measurement means, the variation ofother parameters could be measured across the source 30 and drainelectrodes 31. For example, the stimulation means could assume the formof a high-frequency source which could either stimulate the contacts ofthe SWCNT with a single frequency, or sweep across a wide frequencyrange. This would allow an assessment of the impedance across the source30 and drain 31 electrodes. Similarly, means could be provided formeasuring the resistance, capacitance or inductance.

When considering the provision of detection circuitry, it will berealised that circuitry identical to that used for the measurementelectrodes 34 can also be provided to measure the characteristics of thereference electrode simultaneously.

As shown in FIG. 8, the signals representing variation in conductivity,impedance, resistance, capacitance, or inductance, for example from adetection pair 100 comprising a measurement electrodes 101 and referenceelectrodes 102 can be transmitted to calibration means 105. In the casewhere the signals are analogue, the decision-making means could be ananalogue subtraction. However, it would be possible to digitise thesignals from the measurement and reference sensors using ananalogue-to-digital converter and perform the subtraction between themdigitally. The corrected signal 111 is then output. As mentionedpreviously, a plurality of detection pairs is provided to enable thedetection of an ensemble of biomarkers. It will therefore be appreciatedthat a commensurate number of calibration means will need to beprovided.

A subsequent decision-making means 107 is provided into which thecalibrated signals are input. The decision-making means assesses theensemble of calibrated signals from the plurality of detection pairs,and detects for a condition. If the condition is satisfied, the decisionmaking means indicates this by outputting a logical signal. It will beappreciated that other information relevant to the detection processcould be output, for example raw measurements of conductivity for eachnanotube.

It will be appreciated that the circuitry described above may befabricated on a printed circuit board (PCB), and connected to the sensorsubstrate using bond-wire or some other means. In the preferredembodiment, the sensor substrate is single-use disposable and easilyplugged into, and removed from, a multi-use measurement unit whichcontains the measurement circuitry and output devices. This embodimentenables the sensor substrates to be kept in optimum storage conditions,e.g. sterile environment and temperature control. Alternatively, it willbe appreciated that a proportion of the circuitry could be fabricateddirectly onto the sensor substrate, with the remainder occupying a PCBconnected to the substrate. Additionally, the PCB could be connectedmechanically and electrically to a connector, which for example couldinclude a USE connector, or a micro USB connector, although it will beappreciated that many other types of data connector could be used,including wireless connectors.

Experiments were carried out to test the feasibility of the system, andmeasurements made, as will now be described.

Step 1: Production of the DNA-Aptamer

i) DNA Synthesis

DNA was synthesised using step-wise solid phase synthesis on an ABI 394DNA/RNA Synthesiser using DMTr chemistry protection for 5′-OH and aβ-cyanoethyl-protected 3′phosphite.

Azide functionality was introduced using 3′amino CPG which was thenconverted to 3′azide using azidobutyrate NHS ester chemistry. Analkyne-functionalised “T” phosphoramidite was used to incorporate thealkyne group within the DNA sequence

ii) Purification of Oligonucleotides after Cleavage and DeprotectionUsing HPLC.

Oligonucleotides were purified using either:

-   -   RPLC using a gradient of 100% 50 mM Ammonium Acetate pH 6.8 to        100% 50 mM Ammonium Acetate in 50% acetonitrile, at 55° C.    -   Ion exchange chromatography using water to 100% 1.2M NaCl at 60°        C.

iii) Conversion of 3′-Amino Group to 3′-Azide:

-   -   Azidobutyrate NHS ester was dissolved in MaCN. 3′-amino-DNA        dissolved in 0.1M carbonate/bicarbonate pH 9. Azidobutyrate NHS        ester was added to dissolved oligonucleotide and reacted at room        temperature for 2 hrs. To concentrate 2 volumes of cold ethanol        was added, incubated at −80 C for 20 minutes and pelleted (4,000        g, 30 mins). Redissolved DNA was desalted into 18.2 mΩ water and        lyophilised.

iv) Click Chemistry

-   -   (refer to FIG. 7.)    -   The alkyne-labelled DNA (in water) was added to the azide        labelled DNA to dissolve. CuBr solution containing 0.1 M TBTA in        DMSO/t-butanol 3:1 (v/v) was incubated at 45° C. for 2 hours[1,        2]. Reaction was diluted with water, desalted into 18.2 mΩ water        and lyophilised.

v) Verification of DNA Synthesis and Click Products

-   -   Synthesised DNAs were subjected to negative mode electrospray        ionisation mass spectrometry to verify correct masses.    -   Click reaction products were labelled at the 5′ phosphate using        γ-[32P]-ATP [3]. These were analysed on 10% denaturing SDS        polyacrylamide gel to show successful “click” conjugation.

REFERENCES

-   1. Kocalka, P., A. H. El-Sagheer, and T. Brown, Rapid and efficient    DNA strand cross-linking by click chemistry. Chembiochem., 2008.    9(8): p. 1280-5;-   2. El-Sagheer, A. H. and T. Brown, Click chemistry with DNA. Chem.    Soc. Rev., 2010. 39: p. 1388-1405;-   3. Sambrook, J., Molecular Cloning; A Laboratory Manual. 2nd ed    1989, Cold Spring Harbour: Cold Spring Harbour Laboratory Press.-   Protocols and Reagents-   www.glenresearch.com-   www.linktechnologies.co.uk

Step 2: Oligomer Protection, removal.

-   -   The scaffold or protecting strands were synthesised with a 5′        biotin group enabling their removal post-CNT immobilisation of        the T-piece via the ssDNA GT “foot” fragment by brief (2 min)        incubation above the respective Tm in the presence of        streptavidin-coated magnetic beads. After exposure to a magnet        the supernatant containing the free aptamer strands attached to        the CNTs via the “feet” were used for coating the electrode        elements.

Step 3: Construction of the Reference Electrode.

-   -   Examples of reference electrodes were created by retention of        the aptamer hydrogen-bonded complementary or scaffold strands,        thus blocking formation of the active conformation of the        aptamer stand.    -   Alternatively, the aptamer strands were synthesised with a        photoactive cross-linking group and covalently linked to its        target in the presence of excess target by UV illumination.    -   An initial exaperint fwas performed in order to demonstrate the        uniform dispersion of DNA-coated nanotubes, using frozen herring        sperm DNA solution

FIG. 9 is an atomic force micrograph of such a nanotube dispersion driedon a silica substrate showing the dispersed DNA-coated nanotubes and theformation of a percolated network. The images and line scans confirmthat that the nanotubes are well-dispersed and can dry to form theconductive networks required for the sensor.

Step 4: Dispersion of Nanotubes in (GT)₁₀ in Phosphate Buffer Solution(PBS).

-   -   For making the aptamer-presenting nanotubes, a synthetic DNA,        namely (GT)₁₀, conjugated to a capped aptermr strand, was used.        Frozen (GT)₁₀ solution at a concentration of 3.25 mg/ml was        defrosted at room temperature. 115 microliters of the DNA        solution was sonicated for 15 min.    -   0.19 mg of as-received nanotube powder (Nanointegresis,        semiconducting) was added to 1 ml of PBS and sonicated for 80        min in total using ultrasonic probe using 40 min at 15 W with        additional 0.5 ml of PBS. The nanotube dispersion was diluted to        0.19 mg in 1.5 ml of PBS and sonicated in an ultrasonic bath for        2 h prior to mixing with DNA.    -   The DNA solution (115 microliters) added to the CNTs dispersion        and sonicated in ultrasonic bath. PBS was then added to the        dispersion (285 μl) until the dispersion reached the required        nanotube concentration. Overall, a ratio 2:1 (DNA:SWNT w/w) was        achieved with cDNA:0.2 mg/ml and cCNTs:0.1 mg/ml. The        dispersions were sonicated vigorously in ice water causing the        sample to turn dark red. The dispersions were sonicated for a        total of 2 h, with ice being added to the sonic bath every 20-30        min to prevent the temperature rising above 8° C. Finally, the        dispersions were gently centrifuged at 3300 rpm for 1 h and then        filtered using a 1-micron syringe filter.

Step 5: Deposition of the Nanotube Dispersion onto the Electrodes.

-   -   Electrodes were produced using lithographic techniques. The        electrodes comprised of inter-digitated electrodes as shown in        FIG. 10a . In this drawing, the top diagram is a cross-section        of the part of the inter-digitated electrode design, the middle        is a plan layout of the reference and sensor pair. The top view        is a cross-section of one of the electrodes, while the bottom        diagram is a plan view of a pair of electrodes, one of which        would be used a “reference” electrode and the other as the        “measurement” electrode. The rectangular pads labelled G, S and        D are used to connect the electrodes to the measurement        circuitry. Note that the labelling denoted here is for a        transistor setup with a source, drain and gate contacts. However        the same electrode design maybe used in a non-transistor        situation where the current between the two inter-digitated        electrodes are measured, with or without a gate bias.    -   FIG. 10b is a schematic of an optical micrograph of the sensor        interdigitated electrodes. The gold tracks are 10 microns wide        and spaced by 10 microns.    -   Bare electrodes were rinsed with methanol, acetone and IPA was        blown with N₂ and cleaned in UV-O₃ for 30 min. The electrodes        were isolated on substrates using hydrophobic pen. A 2 μm drop        of CNTs-DNA dispersion was then deposited onto the electrodes.

Results 1: Electrical Measures of Nanotubes-DNA Complex on theElectrodes.

-   -   Semiconducting nanotubes (NanoIntegresis) were dispersed with        single-strand DNA using the protocol discussed with reference to        FIG. 9. Dispersions of 0.1 mg/ml, 0.05 mg/ml, 0.007 mg/ml and        0.001 mg/ml nanotubes were prepared and deposited on the        electrodes. A voltage of 2 V was applied across the electrodes        and the gate voltage varied as the electrode current was        measured.    -   The results are shown in FIG. 11, which has the device current        (left axis) and the absolute device current (right axis) as a        function of gate voltage for various initial dispersion        concentrations. The absolute value of current is V-shaped        because of the bipolar nature of the CNT network. The initial        concentrations in terms of nanotubes are as denoted in the title        of the graphs. It was found that a concentration of about 0.007        mg/ml of nanotubes gives a reliable coverage without excessive        risk of short-circuits. Short-circuits arise from metallic        nanotubes; it is difficult and expensive to ensure pure        semiconducting nanotubes, so the sample used contained about 10%        metallic forms. Shorting was also prevented by having sufficient        difference in the separation of the electrodes compared to the        length of nanotubes used-as as described earlier.

Results 2: Electrical Measures of Nanotubes-(GT)₁₀ Complex on theElectrodes.

-   -   Semiconducting nanotubes (NanoIntegresis) were dispersed with        single-strand DNA using the protocol discussed in Step 4.        Dispersions of 0.1 mg/ml nanotubes were prepared and deposited        on the electrodes as discussed in Example 4. A voltage of 2 V        was applied across the electrodes and the gate voltage varied as        the electrode current was measured.

FIG. 12 shows the device current (right axis, dark line) and theabsolute device current (left axis, light line) as a function of gatevoltage.

Results 3: Electrical Measures of Nanotubes-(GT)₁₀ Complex on theElectrodes.

-   -   In this experiment, aptamers were applied to the DNA.        Semiconducting nanotubes (NanoIntegresis) were dispersed with        single-strand (GT)₁₀-lysozyme aptamer T-piece at using the        protocol discussed in Example 2. These samples are designated        “protected” and still have the conjugated protecting strand        attached to the aptamer. A second set of samples, designated        “unprotected”, were produced by heating the single-strand        (GT)₁₀-lysozyme aptamer T-piece dispersion at 70° C. for 15        minutes prior to introducing them to the nanotube dispersion.        The dispersions were prepared at a concentration of 0.007 mg/ml        and 2 microliters were deposited on the electrodes as described        in Example 4.    -   FIG. 13 shows the device current (right axis, dark line) and the        absolute device current (left axis, light line) as a function of        gate voltage for: LEFT—the “protected” device where the aptamer        is still conjugated to its protective strand, and RIGHT—the        “unprotected” device where the aptamer has had the conjugated        strand removed. The aptamer used in this case was against        lysozyme.

Some examples will now be described of thrombin-specific sensors.

Example 1: CNTs/Deprotected (GT)₁₀ Thrombin Aptamer

Frozen protected (GT)₁₀ thrombin was defrosted at room temperature. 100microliters of this solution was diluted in 900 microliters of PBS togive an approximate concentration of (GT)₁₀ thrombin of 1.2 mg/ml. Thesolution was incubated at 90° C. for 2 min and cooled down slowly atroom temperature to ensure that all the oligomers were base pairedbefore dispersion.

0.6 mg of nanotube powder from Nanointegris was placed into 2 ml of PBSand sonicated using ultrasonic probe for 80 min at 15 W. The CNTsdispersion was sonicated in ultrasonic bath for 30 min prior to mixingwith DNA. 1 ml of the (GT)₁₀ thrombin solution was added to the nanotubedispersion and sonicated in ultrasonic bath. PBS was then added to thedispersion (3 ml) until the dispersion reached the required nanotubeconcentration. This process resulted in DNA:SWNT w/w of cDNA:0.2 mg/ml,cCNTs:0.1 mg/ml.

The dispersions were sonicated vigorously in ice water causing thesample to turn black. The vial was suspended centrally in the bath, at adepth of 20-40 mm, with ice around the edges of the bath preventingheating of the sample.

The dispersions were sonicated for a total of 2 hours, with ice beingadded to the sonic bath every 20-30 min to prevent the temperaturerising above 5° C. Once the nanotubes had been functionalised by the(GT)₁₀ thrombin aptamer, the protecting group was removed by rapidlyheating to 70° C. and harvesting the protecting group using magneticstreptavidin beads.

The dispersions were then gently centrifuged at 3300 rpm for 1 h andthen filtered using a Whatman syringe filter. The dispersions were thendiluted to the required concentration using PBS. In this example thedispersion was diluted to give a concentration of 0.007 mg/ml ofnanotubes.

The electrodes were cleaned and isolated using a hydrophobic pen. A 2microliter drop of dispersion was placed on the electrode and allowed todry. The final devices consisted of a network of nanotubes covered inthe deprotected thrombin aptamer. This electrode is the “measurement”electrode.

The current (ID) between the source and drain electrodes, or rather themain electrodes, was then measured as a function of gate voltage (VD) asshown in FIG. 14. The device shows a semiconducting behaviour. Thedevice is a sensor made by CNTs functionalised by deprotected (GT)₁₀thrombin aptamer. Measurements were made after drying. The currentbetween the electrodes (I, left axis, dark line) as a function of gatevoltage (V). (The right hand axis and light line denote the absolutecurrent (ABS(I)).)

Example 2: CNTs/Protected (GT)₁₀ Thrombin Aptamer

The electrode was made as described in Example 1. However, theprotecting group was left in place so that the final devices consistedof a network of nanotubes covered in the protected thrombin aptamer.This electrode is the “reference” electrode.

FIG. 15 shows the current between the main electrodes (ID) as a functionof gate voltage (VD). As with the deprotected electrode, asemiconducting behaviour was observed. In this device, an electrode madeby CNTs is functionalised by protected (GT)₁₀ thrombin aptamer.Measurements were made after drying. Measurements were made afterdrying. The current between the electrodes (I, left axis, dark line) asa function of gate voltage (V). (The right hand axis and light linedenote the absolute current (ABS(I)).)

Example 3: Comparison of the Response of the Reference and MeasurementElectrodes

Measurement electrodes were produced as described in Example 1 andreference electrodes were produced as described in Example 2. Theelectrical characteristics of the electrodes were measured (FIG. 16).100 nM thrombin was then introduced to the electrodes, allowed to dryand the electrical properties were measured again (FIG. 17). Nosignificant change was observed in the reference electrode after theintroduction of the thrombin, whereas in the “measurement” electrode,the current increased by a factor of ˜4 at 2 V drain voltage.

FIG. 16a shows the as-made reference electrode (nanotubes functionalisedby protected (GT)₁₀ thrombin aptamer) and FIG. 16b shows the as-mademeasurement electrode (nanotubes functionalised by deprotected (GT)₁₀thrombin aptamer). Measurements were made after drying. The currentbetween the electrodes (left hand axis, “I”, dark line) is plotted as afunction of gate voltage (V). (The right hand axis and light line denotethe absolute current, ABS(I).)

FIG. 17a shows the as-made reference electrode (nanotubes functionalisedby protected (GT)₁₀ thrombin aptamer) and FIG. 17b shows the measurementelectrode (nanotubes functionalised by deprotected (GT)₁₀ thrombinaptamer) after the deposition of the 100 nM thrombin and subsequentdrying. The current between the source and drain of the electrode(ID—dark line) is plotted as a function of gate voltage (VD). (Theright-hand axis and light line denote the absolute current.)

Although the invention is anticipated to be useful for detectingtuberculosis, it will be appreciated that it could be adapted to detectbiomarkers indicative of many other conditions as well. Additionally thetechnique may be used to detect molecular species of non-biologicalorigin, as exampled by, but not limited to, chemical-warfare agents,narcotics and explosives. Possible fields of interest are:

-   -   Human/veterinary medicine, in particular infectious diseases        such as human/bovine TB;    -   Homeland security, including biohazards such as anthrax,        tetanus, nerve gases, and explosives such as TNT, 2,4-DNT,        2-6-DNT etc;    -   Law Enforcement, in particular drugs including Cannabinoids,        benzoylecgonine;    -   Health & safety—detection of noxious gases/vapours, naphtha,        hydrocarbons;    -   Process & Quality Control;    -   Very high value products such as fragrances e.g. musk        (galaxolide);    -   General Measurements, Instantaneous or continuous or cumulative:        -   Gas, vapour, liquid, breath, saliva, fingerprint;        -   General or personal environment;        -   Single or multiple measurands.

Other possible applications are:

-   -   Specific diagnostic configurations for detection of Bovine TB;    -   Specific diagnostic configurations for detection of Human TB at        ports of entry;    -   The use of a series of aptamer sensors in parallel to detect a        “biosignature” for TB quantitatively;    -   Specific biomarker—diagnostic configurations for detection of        other diseases (e.g. Hepatitis B and C);    -   Specific diagnostic configurations for detection of biological        warfare agents;    -   Specific diagnostic configurations for detection of explosives;    -   Specific diagnostic configurations for detection of drugs;    -   Specific diagnostic configurations for detection of noxious        gases

The invention claimed is:
 1. A device for identifying the presence of aspecific target molecule or biomarker by the detection of a change in anelectrical property, the device including a measurement sensorcomprising: a conducting or semiconducting sensor structure comprising acoating of aptamers or antibodies capable of conjugating with the targetmolecule or biomarker, thus giving rise to the said change in electricalproperty, and an electrode system for conducting a signal from thedevice; in which the device further includes a reference sensor, ofsubstantially identical form but having the aptamers or antibodies ofits sensor structure capped so as not to conjugate with the targetmolecule or biomarker, in order to act as an internal reference.
 2. Adevice according to claim 1, in which the capping is effected bypre-saturation of the reference sensor structure with the targetmolecule or biomarker.
 3. A device according to claim 1, in which thereference sensor structure contains an oligonucleotide aptamer andcapping is effected by the sensor structure being bound to a complementoligonucleotide.
 4. A device according to claim 1, in which the cappingis effected by using a mutant version of the aptamer with sequencevariations, such that the target molecule or biomarker is no longerrecognized.
 5. A device according to claim 1, in which the measurementand reference sensor structure are made on a semiconductor base coatedwith aptamers capable of conjugating with the target molecule orbiomarker.
 6. A device according to claim 5, in which the aptamers arestereospecific.
 7. A device according to claim 5, the semiconductor baseincludes a Carbon Nanotube (CNT) backbone.
 8. A device according toclaim 7, in which a DNA strand is attached to the CNT backbone, withconjugation molecules attached to the DNA.
 9. A device according toclaim 5, in which the semiconductor base is a DNA backbone coated by amaterial selected from the group consisting of polyvinylpyrrolidone(PVP), aluminum (Al) and silicon (Si) to form a nanowire to giveenhanced measurement of electrical properties.
 10. A device according toclaim 7, in which the electrode system includes a pair of interdigitatedelectrodes between which the measurement and reference sensor structureextends.
 11. A device according to claim 10, in which the electrodesinclude conducting strips including an electrically-conductive materialwith a spacing larger than an average length of the CNTs.
 12. A deviceaccording to claim 11, in which the conducting strips are spaced about10-50 μm apart.
 13. A device according to claim 1, further including asubstrate on which the sensors and electrode system are formed.
 14. Adevice according to claim 13, in which the substrate is a polymerselected from the group consisting of polyethylene terephthalate (PET),polyethylene naphthalate (PEN), polyvinylpyrrolidone (PVP) andpolyethylene glycol (PEG).
 15. A device according to claim 13, in whichthe substrate is silicon.
 16. A device according to claim 1 and havingtwo or more of the measurement sensors, each for a different targetmolecule or biomarker.
 17. A device according to claim 1 and having oneor more of the reference sensors and a plurality of the measurementsensors for multiple measurements of the same target molecule orbiomarker.
 18. A device according to claim 17, in which the measurementsensors are on the same substrate.
 19. A device according to claim 16,where a ratio of the electrical properties measured from the measurementsensor and the reference sensor is used to identify the target moleculeor biomarker.
 20. A device according to claim 1, in which the electricalproperty measured is selected from the group consisting of resistance,conductance, capacitance, impedance, inductance and combinations thereofand is measured using DC or a high-frequency AC signal.
 21. A deviceaccording to claim 5, in which the aptamers are associated with targetmolecules or biomarkers generated by a disease process.
 22. A deviceaccording to claim 1, wherein the device is incorporated into a bodymoulding with appropriate electrical connections, the body mouldingbeing sealed before use in an air-tight manner with a polymer barrierfilm.
 23. A device according to claim 22, further including an interfacebetween the electrodes and a reader device, the reader device measuringthe response of the measurement sensor to provide a judgment ordiagnosis.
 24. A device according to claim 1, further includingcircuitry connected to the electrode system and adapted to compare thesignals from the measurement sensor and the reference sensor to give ameasurement of a concentration of the target molecule or biomarker in asample.
 25. A device according to claim 24, in which the device at apoint of use checks the adequacy of a sample volume including the targetmolecule or biomarker via measurement of electrolyte in the samplevolume before conjugation with the measurement sensor.
 26. A deviceaccording to claim 1, further including software for the automatedcollection and dissemination of data from the device.
 27. A deviceaccording to claim 1, wherein the aptamers are selected from the groupconsisting of deoxyribonucleic acid (DNA) aptamers, ribonucleic acid(RNA) aptamers, and peptide aptamers.